1. Technical Field
The invention is related to improving a signal-to-noise ratio (SNR) of signals captured by sensor arrays, and in particular, to a technique for post-filtering beamformer outputs to provide improved tracking of spatial distributions of signal and noise sources for use in increasing the SNR of array output signals.
2. Related Art
Localization of a signal source or direction within a prescribed region is an important element of many sensor array-based signal capture systems. For example, a number of conventional audio conferencing applications combine the input of microphones from a microphone array using conventional sound source localization (SSL) to enable speech or sound originating from a particular point or direction to be effectively isolated and processed as desired. Signals of other types (radar, radio, sonar, etc.) are also isolated using similar SSL-type techniques.
The ability to combine multiple signals captured from a sensor array is frequently implemented in beamforming systems for providing signal source localization. In general, beamforming operations are applicable to processing the signals of a number of receiving arrays, including microphone arrays, sonar arrays, directional radio antenna arrays, radar arrays, etc.
For example, in the case of a microphone array, beamforming involves processing output audio signals of the microphone array in such a way as to make the microphone array act as a highly directional microphone. In other words, beamforming provides a “listening beam” which points to, and receives, a particular sound source while attenuating other sounds and noise, including, for example, reflections, reverberations, interference, and sounds or noise coming from other directions or points outside the primary beam, thereby providing a higher SNR for sound signals originating from within the target beam.
In general, a beamformer is basically a spatial filter that operates on the output of an array of sensors, such as microphones, in order to enhance the amplitude of a coherent wavefront relative to background noise and directional interference. A set of signal processing operators (usually linear filters) is then applied to the signals from each sensor, and the outputs of those filters are combined to form beams, which are pointed, or steered, to reinforce inputs from particular angular regions and attenuate inputs from other angular regions.
The SNR of the output signal generated by conventional beamformer or SSL-based systems is often further enhanced using conventional post-processing or post-filtering techniques. In general, such techniques operate by applying additional post-filtering algorithms for sensor array outputs to enhance beamformer output signals.
For example, microphone array processing algorithms generally use a beamformer to jointly process the signals from all microphones to create a single-channel output signal with increased directivity and thus higher SNR compared to a single microphone. This output signal is then often further enhanced by the use of a single channel post-filter for processing the beamformer output in such a way that the SNR of the output signal is significantly improved relative to the SNR produced by use of the beamformer alone.
Unfortunately, one problem with conventional beamformer post-filtering techniques is that they generally operate on the assumption that any noise present in the signal is either incoherent or diffuse. As such, these conventional post-filtering techniques generally fail to make allowances for point noise sources which may be strongly correlated across the sensor array. Consequently, the SNR of the output signal is not generally improved relative to highly correlated point noise sources.